In the PON system, communication is performed while synchronizing between an OLT and ONUs so that data in an upstream direction to be transmitted from the ONUs does not collide. The OLT plans to give transmission permission to each ONU so that data in the upstream direction does not collide. At this time, delay due to a distance from each ONU is considered. Therefore, the OLT measures round trip time from each ONU, however, there is a variation of transmission paths, such as jitter and wander, in a transmission by optical fibers, so that measurement needs to be performed periodically.
On the other hand, data communication is not always performed, and, for example during nighttime, data communication is not performed at all. However, measurement of the round-trip time is periodically performed as above regardless of the presence or absence of data communication. Maintaining the ONU in a state capable of constant communication for measuring the round-trip time even when data communication is not performed results in wasting power. Therefore, a technology is studied in which the ONU is intermittently transitioned to a power-saving state by requesting transition to the power-saving state from the ONU.
Moreover, a PON system is studied in which when there is no upstream data from an ONU, useless transmission bandwidth is not allocated to such ONU to improve throughput (Patent Literature 1). In this PON system, when an OLT detects a state in which there is no user data for a preset given period, the OLT deregisters the ONU and notifies the ONU of temporary stop of an optical link. Thereafter, a transmission bandwidth is not allocated to the ONU and transmission of a frame for maintaining the link is also suppressed, so that the ONU can reduce the number of times of transmission of a frame.